Hybrid magnetic materials (Fe 3 O 4 ek-carrageenan) as catalysts for the Michael addition of aldehydes to nitroalkenes Carmen A. Mak a, y, z , Sara Ranjbar a, y, z , Paola Riente a, y , Carles Rodríguez-Escrich a, y , Miquel A. Peric  as a, b, * , y a Institute of Chemical Research of Catalonia, Av. Països Catalans, 16, 43007 Tarragona, Spain b Department de Química Org anica, Universitat de Barcelona, c/ Martí i Franqu es 1-11, 08028 Barcelona, Spain article info Article history: Received 18 March 2014 Received in revised form 11 June 2014 Accepted 16 June 2014 Available online 20 June 2014 Keywords: Hybrid materials Magnetic nanoparticles k-Carrageenan Michael reaction Click chemistry abstract Two hybrid magnetic materials have been prepared from k-carrageenan and Fe 3 O 4 nanoparticles and tested as catalysts for the Michael addition of aldehydes to nitroalkenes. Remarkably, the material prepared from unmodified k-carrageenan showed catalytic activity in the reaction of choice, while the individual components were inactive. This points out to a synergistic effect between the MNPs and k- carrageenan. The second catalyst, bearing a diphenylprolinol silyl ether moiety, was also shown to promote the reaction, giving rise to the corresponding adducts in excellent ees. After the reaction is complete, the catalysts can be conveniently retrieved by simple magnetic decantation. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Organiceinorganic hybrid nanomaterials have generated in- creasing interest in recent years due to their optical and electrical properties and potential biomedical applications. 1 Particularly, nanoparticle-based hybrid materials are very promising systems for a wide range of biomedical applications, including drug delivery. 2 k-Carrageenan (1) is a natural sulfated polysaccharide extracted from different species of edible red seaweed, which is mainly used in the food industry as gelling agent. It is a commercially available material that possesses interesting properties, being nontoxic, mucoadhesive, biodegradable, and biocompatible. k-Carrageenan has also been used as an immobilization support, mainly for bio- catalysts in industrial processes. 3 The combination of these polysaccharides with Fe 3 O 4 magnetic nanoparticles (MNPs) as inorganic supports opens up the possi- bility of exploiting their properties in a synergistic manner, which constitutes an appealing approach to generate new materials for sustainable chemistry. 4 Indeed, precedents of such hybrid nano- structures have been described for various biomedical applica- tionsdmainly as drug delivery systems 5 dor as detoxification agents for the magnetically assisted removal of methylene blue from aqueous solutions. 6 However, to the best of our knowledge, the use of polysaccharide-coated MNPs in catalysis has not been described. Herein, we report the synthesis of such materials and their appli- cation as either catalyst or support for a catalytically active species in the Michael addition of aldehydes to nitroolefins. 2. Results and discussion Before assessing the catalytic ability of the hybrid material we started by establishing a reliable synthetic method that allowed the anchoring of carrageenan onto MNP (2) in two simple steps. First, the MNPs were generated by thermal decomposition of Fe(acac) 3 in the presence of oleylamine and oleic acid as surfactants, following a reported procedure. 7 The transmission electron microscopy (TEM) images show that the MNPs obtained are spherical, mono- dispersed, and small sized (4.80.9 nm; Fig. 1a). The hybrid material was prepared in a very simple manner by mixing k-carrageenan and the MNPs in the presence of glacial acetic acid and ultrapure water in DMF at 110  C(Scheme 1). In- terestingly, the attachment of the polysaccharide did not lead to any significant increase in the size of the MNPs (5.10.8 nm, Fig. 1b). The functionalization level of k-carrageenan in the hybrid nanomaterial, determined by elemental analysis of sulfur, was found to be 0.80 mmol (S) g 1 . The progress of formation of 2 was * Corresponding author. E-mail address: mapericas@iciq.es (M.A. Peric as). y Tel.: þ34 977 920 243; fax: þ34 977 920 244. z These authors contributed equally to this paper. Contents lists available at ScienceDirect Tetrahedron journal homepage: www.elsevier.com/locate/tet http://dx.doi.org/10.1016/j.tet.2014.06.063 0040-4020/Ó 2014 Elsevier Ltd. All rights reserved. Tetrahedron 70 (2014) 6169e6173